When a caterpillar device moves over a surface, such as a surface of the bottom of a ship, formed of a ferromagnetic material coated with materials such paints, there occurs a problem of decrease in a coefficient of friction due to the viscous hydroplaning effect.
The viscous hydroplaning effect differs from the dynamic hydroplaning effect occurring in a vehicle running at a high speed and refers to a water film having thickness of 1/1000 inch or more, which may be formed at a lower speed.
When the viscous hydroplaning occurs, the coefficient of friction can be allowed up to a level of wet ice (0.05 to 0.1).
On the other hand, the viscous hydroplaning may be removed when a high pressure is applied in a short time or time for which a pressure is applied is long. Since applying a high pressure in a short time is likely to do damage to paints applied on the surface of the bottom of a ship, it is preferable to lengthen time for which a pressure is applied.
Under water, when a pressure between the paints on the surface of the bottom of a ship and a wheel is higher than an ambient water pressure, a water film can be discharged.
Examples of a method of attaching a ship cleaning apparatus to the surface of the bottom of a ship and moving the apparatus under water in consideration of the above-described viscous hydroplaning effect may include i) using a form of wheel, (ii) using a form of caterpillar installed with magnets, and (iii) using a form of caterpillar and separate magnets.
In the method of using a form of wheel, since time for which a pressure is applied has to be lengthened to prevent a wheel from being slid on the surface of the bottom of a ship, a ship cleaning apparatus has to be moved very slowly.
In the method of using a form of caterpillar installed with magnets, if a force to push or vertically pull a ship cleaning apparatus occurs, the viscous hydroplaning is not removed since a magnetic force from magnets of the caterpillar is not sufficient to press the viscous hydroplaning.
In the method of using a form of caterpillar and separate magnets, a force to press individual tracks with a roller is irregular and a water film may be again formed in the bottom of a caterpillar due to an external water pressure if the roller does not press the tracks.
A flow velocity in a harbour is 1 knot or less, while a flow velocity in an anchorage out of the harbor is very high, for example, 2.5 knots or so. Most of ship cleaning and inspecting apparatuses using persons or robots are being used at a flow velocity of 1 knot or less.
Since anchorages out of harbor have demands for ship cleaning and inspection, there is a need of a moving apparatus which can be attached to the surface of the bottom of a ship even at a high flow velocity and should satisfy conditions of vertical attachment force and friction simultaneously.
The above description is only provided as a background to assist in understandings of the present invention but is not intended to elucidate the technique known in the art to which the present invention belongs.